Adjustable waveguide filter



April 1964 D. F. BOWMAN ADJUSTABLE WAVEGUIDE FILTER 2 Sheets-Sheet 1 Filed Feb. 13, 1962 N IIIH INVENTOR. 04 Va; awn M4 lane Aw, l-heugGaea 55 Array/rm ATTEA/I/ATO April 21, 1964 ADJUSTABLE WAVEG Filed Feb. 13, 1962 mez-quz/va (Ma) D. F. BOWMAN UIDE FILTER 2 Sheets-Sheet 2 65kg 5 5F1 5 Arraewzys United States Patent Ofifice 3,13%,380 Patented Apr. 21, 1964 3,130,389 ADJUSTABLE WAVEGUIDE David F. Bowman, Wayne, Pa, assignor to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Feb. 13, 1962, Ser. No. 173,040 2 Claims. (Cl. 333-73) My invention relates to a wave signal filter and more particularly to an adjustable waveguide filter constructed of a novel array of intercoupled resonant cavities.

In numerous electronic communication systems the need often exists to selectively couple relatively narrow frequency bands of energy existing within a much broader frequency hand. For example, particular frequency spectrums have been assigned for the various types of communication, such as radio, television, and troposcatter communication, etc. Within such an assigned band spectrum there exists a great number of narrower subbands of signal information (i.e. stations, channels, links). When transmitting or receiving signals within a particular sub-band provision must be included to selectively obtain the desired sub-band while eliminating other subbands within the spectrum of communication. Such signal segregation is normally performed by a tuner or a filter which is designed to transmit the desired pass-band while offering appreciable attenuation to signals outside of the pass-band.

Various filter types are presently known in the prior art. However, none of these heretofore known arrangements possess the advantageous capabilities of the instant invention of permitting a single filter to be readily and conveniently adjusted to any of a number of sub-bands of high power microwave energy. The recent advent of troposcatter communication has presented the need for an adjustable band pass filter to operate at high power levels within the microwave region. Although numerous filters are shown in the prior art for microwave application, none of these exhibit the power and adjustment capabilities of the instant invention. Accordingly, my invention has found particularly advantageous use in troposcatter communications wherein links between various geographical locations on the earths surface have been assigned a different sub-band of channel communication within the overall spectrum. That is, whereas the link from Spain to England might operate between 860 and 866 megacycles, the return link between these two points would be operating between 960 and 966 megacycles. For efficiency of design and operation it is desirable that each stations equipment be standardized to permit operation in either direction of transmission or at any other one of the troposcatter links. This is permitted by the instant invention which allows selective adjustment of the filter band pass anywhere within the overall troposcatter communication spectrum to adapt the equipment to a particular link It is, of course, understood that whereas I speak of, and illustrate, my invention in relation to troposcatter communication equipment the basic concept of my invention permits filter construction at other frequency bands.

Briefly, the filter of my invention comprises a series array of frequency adjustable and selectively intercoupled resonant cavities. As is well known in the art, any volume enclosed by conducting walls possesses a resonant frequency for each particular type of field configuration that can exist in the enclosed volume. Such resonant cavities find extensive use as resonant circuits at microwave frequencies. A resonant frequency of such a resonant cavity corresponds to a possible solution of Maxwells equations, which establish the mathematical criteria for the electric and magnetic fields within the cavity. Each solution corresponds to a particular geometrical configuration of varying electric and magnetic field; each such configuration being known as a mode. The mode having the lowest resonant frequency for a particular cavity is nomenclated the dominant mode. In the case of a prism, the dominant mode corresponds to a guide wave length of twice the length of the prism. Stated otherwise, resonance in the dominant mode occurs when the cavity length equals half guide wave length. Similarly, higher other modes will exist when the length of the prism is equal to any other integral number of half wave length of the impressed energy.

To permit the adjustment of the cavitys resonant frequency, the instant invention has two shorting plungers at the extremities of the prism-like enclosure. By moving the plungers to vary the distance between them the frequency of resonance of the cavity enclosed by their volume may be adjusted.

The coupling of energy to and from the cavities may be performed by many alternative means well known in the art, such as loops, probes, or apertures. In the embodiment of my invention I show aperture openings in the cavity walls, with it being understood that other means of coupling could be used. The degree of coupling between cavities is determined by the location of the inter-cavity coupling aperture with respect to the standing wave pattern of the energy to be coupled. That is, the electric field within the cavity will exhibit zero intensity at tthe shorting plungers and will vary sinusoidally within the cavity. In the preferred manner of operation, wherein the dominant resonant mode is established, only a single half wave length variation will exist. The maximum electrical field intensity is therefore midway between the shorting plungers. Should the intercavity coupling aperture be located at this point of maximum intensity, there will be a maximum intercoupling of energy between the adjacent cavities. The location of the coupling aperture at any position other than corresponding to the maximum point of the standing wave, will result in correspondingly less inter-cavity coupling.

In the instant invention both of the shorting plungers at the opposite ends of the cavity may be moved. Thus, a particular cavity volume enclosed by the end plungers may be longitudinally moved with respect to the fixed coupling aperture to establish an asymmetric relationship. This will present a diiferent portion of the standing wave pattern to the coupling aperture; thereby varying the portion of energy that is coupled between cavities. Such control of inter-cavity coupling permits adjustment of the over-all filter characteristic.

An additional adjustment of the filter characteristic may be provided by the movement of the inter-cavity walls themselves for varying the aperture sizes and locations.

Ideally, it is desirable to obtain a square wave characteristic for the over-all filter response. That is, virtually no attenuation within the desired pass-band of sig nal transmission and an abrupt increase of attenuation immediately outside of the pass-band region. In practice, such an idealized curve is rarely obtained. Each filter is designed to approach the idealized square wave response characteristic to the extent desirable for the particular application. In the instant invention, the necessary number of individual cavities is chosen, and their individual frequency responses and the inter-cavity coupling is adjusted to yield the required filter characteristic.

It is, therefore, a primary object of the present invention to provide a novel wave signal filter of high power capabilities which may be adjusted over a band width appreciably greater than the filter pass band.

Another object of the present invention is to provide a novel intercoupled array of individual waveguide resonant cavities having an overall filter characteristic.

vide a microwave waveguide filter comprised of a series array of individually adjustable resonant cavities wherein the inter-cavity coupling of fixed volume cavities may be adjusted.

'Yet another object of the present invention is to provide a novel microwave waveguide filter comprised of a series array of resonant cavities wherein the volumes of the individual cavities and their placement with respect to cavity inter-coupling apertures may be adjusted by a set of shorting plungers at the cavity extremes.

'It is yet a further object of the present invention to providea novel microwave filter comprised of a series array of intercoupled frequency adjustable resonant cavities wherein the degree of cavity intercoupling may be varied by establishing an asymmetric relationship between the cavity volume and coupling means.

These as well as other objects of the instant invention will readily become apparent on reading the following description of the accompanying drawings:

FIGURE 1 is a perspective view of an embodiment of my invention, with the outer wall partially removed.

FIGURE 2 is a side elevation of the embodiment of FIGURE 1, also with the outer wall partially removed.

FIGURE 3 is an end view of FIGURE 1.

FIGURE 4 is a bottom view of FIGURE 1.

FIGURES 5 .and 6 are side elevations of one of the individual cavities of the waveguide filter taken along line 5-5 .of FIGURE 3 (looking in the direction of the arrows) and showing varying degrees of signal coupling a from. a standing wavepattern of the dominant mode.

FIGURE 7 is a typical frequency characteristic of the waveguide filter of the instant invention.

Referring now toFIGURES 1 through 4, filter 10' contains signal entry and emergent. end waveguides 11 and 12. Inasmuch as filter 10 is a reciprocal device, either one of these openings may be the signal entry end. Waveguides ,11 and 12 are preferably of a standard size of commercially available rectangular waveguide, having the well-known 2:1 aspect ratio between its long and short sides. A number of cavities A, B, C, D, preferably having 7 similar cross-sectional dimensions, are disposed between end waveguides 1 1 and 12 with their respective axis transverse to that of the entry and emergent waveguides. Although four such channels are illustrably shown, the actual number for anindividual filter would be governed by the particular filter response required.

Cavities A, B, C and D are separated by respective in- ,ter-cavitywalls 1313, 14-14 and 15-15. Coupling apertures 16, 17 and 18 located in the inter-cavity walls provide for the coupling of energy between adjacent cavities. Similar apertures 19 and 20 permit the inter-coupling of energy from end waveguides 11, 12 to the series array of resonant cavities. Although coupling apertures are shown, other coupling means, such as loops or probes may also be used. Each cavity A, B, C and D contains a set of adjustable shorting plungers 30, at its extremities. These plungers are shown as the contacting type, having con-tact fingers 40 for improved contact. Although a contacting type'shorting plunger is shown, it is understood that the non-contacting choke type plunger or other shorting plungers well known in the art might be employed. The position of the shorting plungers within the cavity is adjustable by means of an arrangement such as lead screws 31, 31 connected to internally threaded adjusting knobs 32, 32. By turning knobs 32 or 32 the location of shorting plunger 30, and 30' within the cavity may be varied. Lead screws 31, 31 are secured to the ends of cavities A, B, C, D by bars 33, 33, latched to the outer cavity walls at 34, 34'. This convenient arrange ment permits the easy removal of the adjusting plungers. This is only a preferred manner of attachment with it being understood that latches 34,84 may be replaced with a permanent type connection, such as rivets, screws, etc. Alternatively, lead screws 31, 3'1 and knobs 32, 32 may be replaced by a rack and gear arrangement as frequently used in microwave tuner-s.

To adjust the resonant frequency of the dominant mode of an individual cavity the distance between the shorting ends of plungers 30-60" is made to equal one-half the guide wavelength of that frequency. Also, assuming a given volume defined by end plungers (30, 30) and a fixed location for the aperture openings (16, 17,18, 19, 20) their relationship may be varied by the movement of both end plungers 30, 30'. That is, end plungers 30, 30 permit both an adjustment in the volume of the defined cavity, and the asymmetric location of the defined cavity with respect to the coupling aperture. As will be set forth below, the relative position of the coupling aperture and the resonant cavity volume controls the degree of inter-cavity coupling.

Reference is now made to FIGURES 5 and 6 which illustrates the manner in which variations of inter-cavity coupling is achieved. In those figures, sinusoidal waveform 35 illustrates the voltage distribution of the electric field. The placing of shorting plunger 30 at one of the cavity extremes creates a short or zero voltage point at that point. Likewise, shorting plunger 30' creates a zero voltage point of the other cavity extremity. When the impressed signal energy has a half wave-length corresponding to the distance between plungers 30, 30 a resonant condition will be set up as shown in these figures. Although I show only one (1) half wave-length of variation, a similar condition will exist for higher modes when the cavity lentgh equals any other integral number of half wave-lengths of the impressed energy. However, in the preferred dominant mode as illustrated, there is a single half wave-length of variation within the cavity.

The portion of energy coupled between associated cavities is a direct function of the voltage across the particular inter-cavity coupling aperture. Hence, maximum coupling between cavities B and C will be achieved when aperture 17 is symmetrically located midway between the plungers 30-30' as shown in FIGURE 5. The coordinated movement of plunger set 30-30', while maintaining the cavity B volume, destroys the symmetry as shown in FIGURE 6. Since aperture 17 is now .presentedto a lesser voltage point, a correspondingly lesser portion of energy will be in-tercoupled between cavities .B and .C. Thus, it is seen that the pair of adjustable shorting plungers 30, 30' located at the cavity extremes may be adjusted to permit a given volume enclosed ,by such plung ers to be repositioned with respect to the intercoupling aperture. This provides an adjustment of the coupling between adjacent cavities, which in turnpermits increased control of the filter response.

To permit a greater rangeof coupling adjustment, the inter-cavity walls 13-43, 1414 and ,15 15' may also be made adjustable to vary the sizeor location of the inter-cavity coupling apertures (16, 17, 18). In particular, the staggering of aperture openings 16, 17, 18 would permit the variation of coupling between certain of the cavities A, B, C, D without efiecting the coupling between other cavities. Likewise, end walls 36 -36 and 3737 may also be made adjustable to vary the size or location of the signal and emergent apertures (19, 20).

FIGURE 7 illustrates an exemplary response characteristic of the filter of the instant invention. Typically,

this filter might be designed ,to operate within the 755 to 985 megacycle frequency spectrum, presently assigned for troposcatter communications. A fairly narrow pass band of approximately 6 niegacycles, as shown by region a may be achieved by proper adjustment of the shorting plunger sets 3030. Likewise, the volumes defined by shorting plungers 3-030 may be altered to achieve a similar pass band in regions b, c, a, or any other such region within the broad spectrum. Each of the sub-bands a, b, c, d, would correspond to a different troposcatter link.

The overall filter approximates the Chebishev response characteristic commonly associated with a series of resonant circuits. The three basic parameters of such a characteristic are (a) the rate at which the attenuation increases outside the desired pass-band, (b) the amount of attenuation within the pass-band, and (c) the amplitude of the ripple within the pass-band. Each of these parameters are determined by the characteristics of each individual cavity, the portion of energy coupled between cavities, and the number of cavities comprising the array. Hence, the requirement of a particular application would determine the number of individual cavities of the series array; with an increase in the number of cavities permitting a closer approximation of the ideal square wave response.

To facilitate the tuning of the filter it is desirable to tune one cavity at a time to the approximate resonant frequency. T 0 permit this, it is convenient to remove the effect of its adjacent cavity during the initial tuning period. This may be provided for by openings 38 wherein shorting posts may be inserted. The insertion of such a post completely across the guide will act as an inductance and isolate adjacent channels during the tuning period. Naturally, when the post is removed the adjacent channel has some de-tuning effect on the previously tuned channel. Minor adjustments would then be made to compensate for such effects. Also, the presence of openings 38 permits the insertion of capacitive tuning posts, should such be necessary in particular applications.

Although I have described preferred embodiments of my novel invention, many variations and modifications will now be obvious to those skilled in the art, and I prefer therefore to be limited not by the specific disclosure herein but only by the appended claims.

The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:

1. A microwave filter comprising at least a first, second and third cavity enclosure; said first cavity enclosure being operatively connected to a first guide means and said third cavity enclosures being operatively connected to a second guide means; each of said first and second and third cavity enclosures having a first end surface, a second end surface, a first wall surface, and a second Wall surface; said second wall surfaces of said first cavity and said first wall surface of said second cavity coinciding to form a first common wall between said first and second civity; said second wall surface of said second cavity enclosure and said first wall surface of said third cavity enclosure coinciding to form a second common wall between said second and third cavity enclosures; each of said cavity enclosures having a resonant frequency adjustably preselectable by varying the distance etween their respective first and second end surfaces; said first wall surface of said first cavity having a first coupling means; said first common wall having a second coupling means; said second common wall having a third coupling means; said second wall surfaces of said third resonant cavity having a fourth coupling means; said first coupling means being operatively positioned to supply electromagnetic energy to said first cavity enclosure; said second coupling means being an outlet for a first portion of said electromagnetic energy to be intercoupled to said second cavity enclosure; said third coupling means being an outlet by a second portion of said electromagnetic energy to be intercoupled to said third cavity enclosure; each of said cavity enclosures having a first and second adjusting means; each of said adjusting means being operatively connected to one of said end surfaces to permit relative movement thereof; said end surfaces of said first cavity enclosure being operatively positioned to define a first resonant condition; said end surfaces, while defining said first resonant condition, being selectively positioned with respect to said second coupling means to control said first portion of electromagnetic energy; said end surfaces of said second cavity enclosure being operatively positioned to define a second resonant condition; said end surfaces while defining said second resonant condition being selectively positioned with respect to said third coupling means to control said second portion of electromagnetic energy; said fourth coupling means being an outlet for a third portion of said electromagnetic energy; and said end surfaces of said third cavity being operatively positioned to define a third resonant condition; said end surfaces, While defining said third resonant condition, being selectively positioned with respect to said fourth coupler means to control said third portion of electromagnetic energy; the frequency response of said first, second and third cavity enclosures and said controlled intercavity coupling collectively defining a predetermined filter characteristic.

2. The microwave filter of claim 1 wherein at least one of said first and second common Walls is comprised of a plurality of adjustable wall sections separated by a coupling means, and said last mentioned coupling means is an aperture opening.

References Cited in the file of this patent UNITED STATES PATENTS 2,106,768 Southworth Feb. 1, 1938 2,432,093 Fox Dec. 9, 1947 2,510,288 Lewis June 6, 1950 2,513,761 Tyson July 4, 1950 2,518,092 Sunstein Aug. 8, 1950 2,540,488 Mumford Feb. 6, 1951 2,549,443 Fiske Apr. 17, 1951 2,588,103 Fox Mar. 4, 1952 2,626,990 Pierce Jan. 27, 1953 2,694,186 Kinzer Nov. 9, 1954 

1. A MICROWAVE FILTER COMPRISING AT LEAST A FIRST, SECOND AND THIRD CAVITY ENCLOSURE; SAID FIRST CAVITY ENCLOSURE BEING OPERATIVELY CONNECTED TO A FIRST GUIDE MEANS AND SAID THIRD CAVITY ENCLOSURES BEING OPERATIVELY CONNECTED TO A SECOND GUIDE MEANS; EACH OF SAID FIRST AND SECOND AND THIRD CAVITY ENCLOSURES HAVING A FIRST END SURFACE, A SECOND END SURFACE, A FIRST WALL SURFACE, AND A SECOND WALL SURFACE; SAID SECOND WALL SURFACES OF SAID FIRST CAVITY AND SAID FIRST WALL SURFACE OF SAID SECOND CAVITY COINCIDING TO FORM A FIRST COMMON WALL BETWEEN SAID FIRST AND SECOND CIVITY; SAID SECOND WALL SURFACE OF SAID SECOND CAVITY ENCLOSURE AND SAID FIRST WALL SURFACE OF SAID THIRD CAVITY ENCLOSURE COINCIDING TO FORM A SECOND COMMON WALL BETWEEN SAID SECOND AND THIRD CAVITY ENCLOSURES; EACH OF SAID CAVITY ENCLOSURES HAVING A RESONANT FREQUENCY ADJUSTABLY PRESELECTABLE BY VARYING THE DISTANCE BETWEEN THEIR RESPECTIVE FIRST AND SECOND END SURFACES; SAID FIRST WALL SURFACE OF SAID FIRST CAVITY HAVING A FIRST COUPLING MEANS; SAID FIRST COMMON WALL HAVING A SECOND COUPLING MEANS; SAID SECOND COMMON WALL HAVING A THIRD COUPLING MEANS; SAID SECOND WALL SURFACES OF SAID THIRD RESONANT CAVITY HAVING A FOURTH COUPLING MEANS; SAID FIRST COUPLING MEANS BEING OPERATIVELY POSITIONED TO SUPPLY ELECTROMAGNETIC ENERGY TO SAID FIRST CAVITY ENCLOSURE; SAID SECOND COUPLING MEANS BEING AN OUTLET FOR A FIRST PORTION OF SAID ELECTROMAGNETIC ENERGY TO BE INTERCOUPLED TO SAID SECOND CAVITY ENCLOSURE; SAID THIRD COUPLING MEANS BEING AN OUTLET BY A SECOND PORTION OF SAID ELECTROMAGNETIC ENERGY TO BE INTERCOUPLED TO SAID THIRD CAVITY ENCLOSURE; 